We are continuing to immunize goats with leukocyte pyrogen to raise antisera for use in a radio-immunoassay. We have shown that macrophage pyrogen is distinct from neutrophil pyrogen in its iso-electric point and now wish to show that macrophages make the same pyrogens in response to stimulation by myxoviruses, endotoxin and staphylococci. We also wish to determine whether macrophage pyrogens share the non-pyrogenic activities of neutrophil pyrogen such as stimulation of fibrinogen synthesis and lowering of serum iron. We have suggestive evidence that macrophage pyrogen is made de novo from aminoacid precursors and wish to extend these results. Using antibodies raised against neutrophil pyrogen, we have shown that macrophage pyrogen cross-reacts, but is not identical with neutrophil pyrogen. We wish now to explore just how the two pyrogens differ.